Chemical wood-pulping process



' Feb. 13, 1940.

G. A. RlcH'rER A2,190,193

CHEMICAL WOOD-PULPING PROCESS Filed Jan. 3l, 1936 Patented Feb. 1940 George A. Richter, Berlin, N. H.. assignor yto Brown Company, Berlin, N. H., a corporation o! Maine Application January 31,1936, serial No. 61,618 s claims. (ci. :i2-1s) This invention relates to various chemical Wood-pulping processes and more particularly to those wherein a cooking liquor containing caustic soda is employed for pulping chippedwood. Its objective is the pulping of chipped wood under` conditions such that the resulting pulp excels that produced by the usual processes in tear resistance, bleachability, and other respects. l -In producing chemical wood pulps it is sometimes the practice to use cooking liquors contain-v ing caustic soda as an essential or principal fiberliberating chemical. Thus, so-called kraft or sulphate pulp is produced by cooking the raw chipped Wood in a liquor wherein caustic soda and liberating chemicals. Again, so-called soda pulp derives its name from the caustic soda-containing cooking liquor in which it is produced, namely, a liquor containing practically only caustic soda as the active pulping chemical.. And there are modications of these processes based on the use of liquors containing caustic soda and sodium sulphite or containing caustic soda, sodium sulphite, and sodium sulphide as the pulping agents. All of these processes, however, are performed similarly in the respects that the cooking is effected with an amount of liquor only moderately in eX- cess of that required to cover the chips and con- 3 taining uber-liberating chemical only moderately in excess of that consumed or spent by reaction with the ligneous or encrusting components oi the Wood.

In accordance with the present invention, wood 35 pulp is produced by cooking appropriately chipped wood in a sodium base alkaline liquor of the character hereinbefore described in volume more than twice that requisite for covering the wood and containing fiber-liberating chemical more .than 40 twice that requisite forcomplete pulping or vber liberating action,- for I have found that wood pulp thus produced is of notably betterquallty than the pulps heretofore produced.` It is thus seen that the process hereof involves cooking 45 Wood in a liquor whose active sodium content based on wood is much greaterI than that of the liquors heretoforeused but whose active. sodium concentration is not necessarily in excess of that' of the liquors heretofore used. Indeed, in some 50 instances, the cooking liquors ofthe present invention may be no more concentrated or even less concentrated in active sodium constituent than the liquors heretofore used even though they are associated in such volume with the wood as 55 to contain more than twice the amount of active sodium sulphide serve as the essential iiber-I sodium constituentnecessary to accomplish complete pulping or ber liberation.

While it is possible to embody the principles of the present invention in various specific pulping' lprocesses performed with sodium base alkaline 5 liquors, I shall now illustrate how these principles may be applied advantageously in the pulping of wood by a so-called kraft cooking liquor containing both caustic soda and sodium sulphide as the ber-liberating chemicals. Spruce chips, such as 10.

are ordinarily prepared for kraft cooking, 'are put into a digester together with a suilicient volv ume of kraft cooking liquor containing 2 pounds of NaaO equivalent per cubic foot to provide a digester charge containing 0.4 pound of NanO 15 equivalent per pound of bone dry wood. The liquor may be one whose sodium base exists essentially in the form of caustic soda and sodium sulphide, about to 50% of its NaaO content, for instance, being in the form of sulphide. 'I'he 20 digester charge so made up, which includes a volume of liquor more than twice that necessary to cover the wood chips, may be cooked for about 4 hours at 335 F., at the end of which time the chips are fully pulped and the digester contents 25 maybe discharged into a blow tank wherein the liquor may be drained or otherwise .separated from the pulp. 'I'he resulting pulp upon being washed has a so-called cooking stain value of about 115, which value is distinctly higher than 30 that of the ordinary commercial kraft pulp and' indicates that the pulp is of distinctly lower ligneous content and may be bleached more readily and with lower bleach consumption than the ordinary commercial Akraft pulp. When the pulp is beaten to a predetermined degree and formed into test sheets of paper, it lexhibits a tear resist'- ance of at least about 300 as compared with va tear resistance of only about 250 in the case of the usual kraft pulp similarly tested. It might be ob- 40 served that the foregoing comparison is based on pulps derived from a particular kind of wood,

5 namely, a Northern spruce which was' reduced to a particular chip size preparatory to pulping. Although other woods, such as Southern pine, Western nr, white birch and other hardwoods, etc., might yield pulps qf diiferent tear( resistance values, such other woods when pulped bythe process hereof yield in all cases pulps of de tributable to the large excess of both cooking liquor and liber-liberating chemical employed in the pulping of the wood. Thus, these results cannot be approached even when, as is true in the usual kraft process, the liquor contains a greater concentration of active sodium constituent, that is, is stronger, but is used in volume, relative to wood, muchv less than that used in the process hereof. For instance, even when a kraft liquor contains as much as 3 pounds of NazO` equivalent per cubic foot of liquor, it yields wood pulp distinctly inferior to that produced hereby when it is associated or used with raw wood in such limited volume that the NazO equivalent amounts `to, say, 0.2 pound per pound of wood, as may be true in the usual kraft process. Onvthe other hand, when only the ratio of fiber-liberating chemical to wood is increased, the improved pulp product hereof is not realized. Thus, when pulping of wood is accomplished in the usual volume of kraft cooking liquor, even though the liquor may be so concentrated as to contain 0.4 pound of NazO equivalent per pound of dry wood, it does not yield the product hereof. On the contrary, the resulting pulp generally has a tear resistance evenlower than that of the usual kraft pulp.

In the case of kraft liquors, a marked improvement is effected in the quality of the pulp when cooking of the wood takes place in a liquor whose active NazO content ranges from about 2 to 3 pounds per cubic foot and whose volume relative to wood is such that at least about 0.3 to 0.6 pound of active Na2O equivalent is present per pound of `dry wood. These same active' NazO values may be advantageously maintained in socalled soda liquor, that is, liquor wherein essentially all of the active sodium constituent is present as caustic soda, or in liquor wherein, besides caustic soda, part of the active sodium constituent exists as sodium sulphite or as sodium sulphite and sodium sulphide.

In the particular example hereinbefore described, wood chips of the` usual size were cooked or pulped, that is, chips of about 3A to 1" length, about to 1/,."'width, and about V8 to 1A" thickness. However, it is possible to depart widely from such particular chip dimensions or size while pursuing the practice of the present invention. For instance, it is possible to chip wood into much larger units or to comminute wood into fragments approximating sawdust and to pulp these various reduced forms of wood by the process hereof. When the objective hereof is a finished pulp possessed of high average liber length and good tear resistance as well as easy bleachability, it is preferable to use wood chips of the usual size or even larger, but, in .those instances when the objective is a uniformly cooked pulp of easy bleachability, smaller than normal wood units or fragments may advantageously be the form of cellulose subjected to the pulping operation. It is to be understood,I therefore, that the expression chipped wood as used in the appended claims is meant to be inclusive of wood chips or fragments of various sizes that may be pulped by the process hereof. y

In practicing the process of the present invention, it is unnecessary to provide digesters of such size as to hold all the pulping liquor to vact upon the wood chips being pulped. Thus, it is possible to provide a tank or reservoir outside of the digesters for the purpose of holding theliquor` in excess of that normally used and to cause such excess liquor while at the desired cooking temperature to circulate through the digesters as cooking ofthe chips is taking place. I have shown diagrammatically in the accompanying drawing a system or arrangement of instrumentalities such as may be adopted in the practice of the present invention.

The numerals I represent a battery of digesters in which chipped Wood may be cooked or pulped under the conditions hereinbefore described. After the pulping operation, the digester contents may be deliveredinto a series of blow tanks II, wherein the liquor may be separated, as by draining, from the pulp. The pulp may be sluiced from the blow tanks I I to a suitl able washer, for instance, to a. countercurrent Washer that dilutes the residual liquor washed from the pulp to a minimum degree so that the valuable sodium constituent may be economically recovered from the liquor for reuse.

The liquor drained or otherwise separated from the pulp in the tanks II is rich in unconsumed or active pulping chemical and may hence be reused in the pulping of raw Wood. Thus, Whereas in the usual kraft process, the black or spent liquor contains largely spent alkali or NazO equivalent, the sodium content of the used liquor of the present invention is present largely as active or residual alkali. Such used liquor may be passed to a preheating tank I2 wherein it may be commingled with fresh alkali or so-called White liquor from a supply tank I3 in amount suilicient to increase its active alkali content to that desired for the cooking of fresh wood. After the used liquor has been brought to the desired active alkalinity, it may be heated under pressure to the temperature, say, 335 F., to be maintained during cooking in the digesters I0, as by circulation from the closed tank I2 .through a heater Il back to such tank.

The liquor preheated to the desired temperature in the closed tank I2 may be passed to a closed accumulator tank I5 having a vent line I6 communicating with the tank I2 so that air or steam to be displaced in the accumulator tank may be released into the closed tank I2. After' the preheated liquor has been exhausted into the accumulator tank I5, the valves in the pipe lines connecting this tank with the preheating tank I2 may be closed and the air or steam in the rpreheating tank I2 may be released preparatory to running thereinto other used liquor to be preheated. As soon as there is accumulation' of the desired amount of preheated liquor in the tank I5, the liquor may be pumped therefrom into the digesters through their bottom ends so as to submerge or cover their charges of raw chipped wood. The cooking of the chipped wood in the digesters takes place as liquor is withdrawn from the digesters at their upper ends and is passed to the accumulator tank I5 which may be provided with steam-heating coils or other suitable heating means adapted to keep the liquor at the desired cooking temperature, for instance, 335 F. As cooking of the chipped wood in the digesters proceeds, liquor is constantly pumped into the lower end portions of the digesters, and circulated back to the tank I5 which contains therein liquor in suflicient volume to produce the improved pulping action on the wood chips desired herein. In other words, the total volume of .liquor in the system, that is, the volume of liquor in the digesters and in the tank I5, is more than twice that necessary for covering the chips in all the digesters. course, the withdrawal of liquorat the top of digesters is eected at such a rate that the ofl ' since heating of practically all the excess liquor necessary for the purpose of the present invention may be done outside of the digesters while it is out of contact with the wood chips. It v'thus becomes possible to practice the present invention without building excessively large digesters of such strength or structural design as to be capable of withstanding the weight of both the chipped wood and all of the liquor participating in the pulping reaction. In other words, the large volume of excess liquor outside of thev digesters is constantly maintained in circulatory contact with the wood being cooked in the digesters and thus conduces to substantially the same results as if all the liquor were present with the chipped wood in the digesters.

When cooking of wood is carried out with soda liquor or liquors containing caustic soda and sodium sulphite or caustic soda sodium sulphite and sodium sulphide, the very same system as that herein described and illustrated may be employed. In all such cases, the liquor should contain more than twice the amount of ber-liberating chemical necessary to ensure complete pulping of the wood, but such chemical, rather than being present in the usual volume of liquor, is dissolved in a volume of liquor more than twice that necessary for covering the chips. It might be observed that the improved results of the present invention may be attributable to the fact that the concentration of active fiber-liberating chemical in the liquor does not undergo marked change during the cooking operation; and it is for this reason that the used liquor, that is, the liquor separated from the completely pulped chips, contains fiber-liberating chemical largely in active condition. Apparently, in the ordinary cooking processes the fiber is evidently injured toward the end stages of the cooking or pulping operation by reason of the fact that, as `the concentration of unconsumed caustic soda and/or other fiber-liberating chemical is markedly reduced, the liquor tends to hydrolyze the ber. On the other hand, in thecooking operation hereof, there is comparatively much smaller change in the concentration of the caustic soda and/or other liber-liberating chemical as the cooking operation proceeds so that cooking or ber-liberating action at the end stage of the cooking operation is about as intensive as at the early stage of the cooking operation and a more uniformly cooked pulp is realized in a given period of time; and, since the cooking liquor at the end of the cooking operation is quite rich in unconsumed caustic soda or other liber-liberating chemical, there is less injury done to the fiber at the end-stages of cooking by reason of liberhydrolysis.

The process of the present invention is ap.

plicable to various kinds of woods, such as spruce, fir, pine, birch, maple, etc. In other words, soitwoods and hardwoods of all sorts may be pulped as herein described.

I claim: v

1. A process of pulping lchipped wood in an alkaline sodium base cooking liquor containing caustic soda as an essential fiber-liberating chemical, which comprises cooking the chipped wood in a plurality of digesters each of which contains a volume of such liquor not much more than that necessary to co'ver said wood, said liquor having an active sodium constituent concentration, including caustic soda, from substantially the very beginning of the cooking operation amounting to about two to three pounds oi NazO equivalent per cubic foot of liquor; and, during the cooking operation, maintaining circulatory communication between the liquor in said digesters and a sufficiently large reservior of similar liquor out- 'side of said digesters to cause a volume of liquor more than twice that requisite for covering the wood and corresponding to at least 0.3 to 0.6 pound of active NaaO equivalent per pound of dry Wood. to participate in the cooking operation.

2. A process of pulping chipped wood in a cooking liquor containing caustic soda and sodium sulphide as its essential fiber-liberating chemicals, which comprises cooking the chipped wood in a plurality of digesters each of which contains a volume of such liquor not much more than that necessary to cover said wood, said liquor Y toy 0.6 pound of NazO equivalent per pound of dry wood to participate in the cooking operation.4

3. A process of pulping chipped wood in an alkaline sodium base cooking liquor containing caustic soda as an essential liber-liberating chemical, which comprises cooking the chipped wood in a digester which contains a volume of such liquor not much more than that necessary to cover said wood; and maintaining during the cooking operation circulatory communication between theliquor in said digester and a suiciently large reservoir of similar liquor outsideof said digester to cause a volume of liquor more than twice that requisite for covering said wood to participate in the cooking operation, the total volume of liquor participating in the cooking operation containing far more than sumcient active liber-liberating chemical to pulp said Wood completely.

4. A process of pulping chipped wood in an alkaline sodium base cooking liquor containing caustic soda as an essential fiber-liberating chemical, which comprises cooking the chipped wood in a digester which contains a volume -of such liquor not much more than that necessary to cover said Wood; maintaining during the cooking operation circulatory communication between the liquor in said digester and a sufciently large reservoir of similar `liquor outside of said digester to lcause a volume of liquor more than twice containing far more than suiilcient active berliberatingchemcal to pulp said wood completely; and heating the liquor While part of said reservoir outside of the .digester substantially to the 5\temperature of cooking in said digester.

5. A process o1' pulping chipped wood in an alkaline sodium base cooking liquor containing caustic soda as an essential iibe-liberating chemical, which comprises cooking the chipped Wood in a digester which contains a volume of suchliquor not much more than that necessary to cover said Wood, said liquor having an active sodium constituent concentration, including caustic soda, amounting to about 2 to 3 pounds of NazO equivalent per cubic foot of liquor; main= taining during the cooking operation circulatory communication between the liquor in said digester and a suiiiciently large reservoir of similar liquor outside of said digester to cause a volume of liquor more than twice that requisite for covering the wood and corresponding to at least '0.3 to 0.6

pound of active NazO equivalent per pound of dry wood to participate in the cooking operation; and heating the liquor While part of said reservoir outside of the digester substantially to the temperature of cooking in said digester.

6. A process of pulping chipped wood in a cooking liquor containing caustic soda and sodium sulpliite` as its essential fiber-liberating chemicals, which comprises cooking the chipped wood in a plurality of digesters each of which contains a volume of suchliquor 'not much more than that necessary to cover said Wood, said liquor oontaining caustic soda and sodium sulphide in concentration, calculated as NaeO equivalent, amounting to about 2 to 3 pounds per cubic foot of liquor; maintaining during the cooking operation circulatory communication between the liquor in said digesters and a suciently large reservoir of similar liquor outside of said digesters to cause a volume of liquor more than twice that requisite for covering the wood and corresponding to 0.3.

to 0.6 pound of NazO equivalent per pound of dry wood to participate in the cooking operation; and heating the liquor while part of said reservoir outside of the digesters substantially to the temperature of cooking in said digesters.

GEORGE A. RICHTER. 

